1. Field of Invention
This invention relates to electrically controlled rotary positioning systems, more particularly to a stepper motor positioning servo loop.
2. Description of the Prior Art
There are many known applications in which electric signals cause positioning of mechanical devices, particularly rotary devices. One example is in the actuation of rotary valves in engine fuel control systems, in which a fuel schedule is converted into a required valve position. For accuracy, it is common to take positioning information from the mechanical device and feed it back for combination with the demand signal so as to drive the mechanical device with an error signal, in a typical, known servo loop. In some applications, it is desired to have fail safe operation; that is, should power be lost, position of the mechanical device would not be changed; therefore, it is frequently desirable to use a known stepper motor to rotate a mechanical device, rather than a torque motor or other rotary electromechanical transducers. However, to provide feedback from a stepper motor requires the use of an analog positional feedback sensor, or requires tracking of the position caused by the stepper motor. When tracking is important, some form of counter, which may comprise a portion of a digital processor which generates the positional demand signals, must remember the position which should have been attained by the mechanical device in response to the commands given to the stepper motor; but this is open-loop tracking and is subject to errors resulting when there is a failure of response for one reason or another; and it is generally subject to volatility--that is, if power is lost, the tracking information stored in memory is also lost. This form of tracking is therefore not useful in airborne applications which demand that a system be able to tolerate power outages extending over a duration of seconds or more, or in critical applications (such as control of engine fuel valve) in which failure to properly track in an open-loop fashion could be catastrophic. On the other hand, rotary position sensors, such as potentiometers, or rotary variable displacement transformers, resolvers and the like, are generally quite temperature sensitive and produce large errors over moderate temperature ranges; when the position being sensed is adjacent to a widely varying heat source, such as an aircraft engine, the errors induced can be prohibitively large, or very sophisticated sensors with or without additional temperature compensation can render the cost of such positional sensors prohibitively high for any applications.
A particular example of the foregoing considerations is found in missiles having air breathing engines. In some applications they may be launched at sea level and in others they may be launched from very high altitudes (such as from an aircraft). The difficulty in starting such an engine is related in part to the accuracy of fuel flow provided as a function of ambient pressure; starting, of course, is done with relatively lower temperatures in the ambient surrounding the engine; but thereafter, guidance and control of the missile include controlling of fuel in a rather precise fashion at elevated temperatures. Due to the fact that the missile is a self-destructing weapon, the fuel control system is used only once and is then naturally destroyed upon detonation of the missile. Therefore, both low cost and a certain degree of accuracy are required.